Numerous methods and apparatus exist in the art for measuring chemical components of fluids. For instance, when the fluid is a liquid or liquid with a dissolved gas with or without the presence of solids, it may be necessary with current technology to transport a sample to a location for testing. With centralized testing, the bulky, stationary, elaborate and sophisticated equipment performs the analysis on a practically endless number of samples. An example of this is the qualitative and/or quantitative measurement of constituents or analytes of blood. For instance, the measurement of blood gases, usually a measure of the partial pressures of oxygen and carbon dioxide, along with the pH from a sample of arterial blood gives the state of the acid base balance or the effectiveness of both the respiratory and cardiovascular systems of the human or vertebrate body. For measuring constituents of blood, the blood sample is drawn from the patient and usually, as in the case of blood gases, transported to a central location for testing.
This technique of transporting the sample to stationary measuring equipment can lead to problems. Ingenious technology has broached solutions to maintain the original composition of the fluid during transportation. Elaborate designs for syringes used in taking the blood samples overcame some problems that resulted in inaccurate readings of the particular chemical constituent being measured. For instance, for determining blood gas composition, the problem of air contamination in the collected sample was solved by the use of liquid heparin as an anticoagulant. Unfortunately, this introduced a sample dilution problem. Subsequent development resulted in the use of heparin in the dry state as opposed to the liquid state to avoid this dilution. Also, elaborate designs are provided for proper mixing of the sample after transportation but before testing. Even with these improvements, there are many reports in the literature that suggest that the values obtained in the measurement of blood gases depend on the type of measuring equipment and the technique for sample collection.
The art also has attempted to develop more portable measuring equipment rather than the fairly expensive nonportable equipment that engender the elaborate and cumbersome transportation techniques. Devices that are very portable could shorten or overcome transporting the sample altogether so that a patient's blood gases could be measured at the bedside in a manner similar to measuring a patient's temperature. U.S. Pat. Nos. 3,000,805 and 3,497,442 show two such devices. The former has electrodes located on a syringe plunger and the latter has electrodes placed on the syringe well to conduct the measurements. The electrodes are the sensing devices for the blood gases. In the allowed U.S. patent application Ser. No. 07/343,234, Applicants' assignee describes and claims a portable blood gas sensor which includes electrodes fabricated from a conventional silk screening process where the electrodes are screened on to a ceramic substance. Typically, these electrodes are used along with an electrolyte and analyte permeable membrane that covers the sensor. Some of these membranes may be hydratable membranes that can be stored in a dry state and hydrated just prior to use.
It is an object of the present invention to provide a sensor assembly apparatus that utilizes at least one hydratable membrane that can be useful in portable measuring devices or can be placed in catheter lines or actually utilized with stationary equipment where the apparatus allows for the hydrated state of the membrane. This gives the advantages of: ready-to-use sensors, establishment of a stable electronic operation with stable potential for potentiometric type sensors and maintenance of electrolytic contact between electrodes in amperometric sensors, electrolyte present for reference electrodes, and/or reduced voltage drift like that experienced during a hydration step for dry sensors with hydratable membranes.
Placement of the all of the components, including the heater, on the wiring board can result in the maximum utility and capability of these components and minimize power consumption.